In the past, ballasts were developed which converted input alternating current power into direct current power and then into high frequency alternating current power for causing the lamp to emit light. In the aircraft industry, significant weight advantages were realized because high frequency transformers and inductors are lighter than comparable low frequency versions.
However, with these ballasts, the voltage would have to be shaped to a sine wave with an L-C tank circuit and the current would generally have to be limited by an inductive device both of which would result in poor magnetic volt-ampere utilization.
Further, an energy storage, hold-up capacitor was required on the direct current side of the input rectifier to smooth out the rectified sine wave so as to avoid pulsations in the fluorescent light emission. If the capacitor were large, the input conduction angle would be reduced causing a poor power factor, large peak input current, and significant turn-on surge currents. If the capacitor were small, a larger transformer was required to provide greater boost and drop, and the lamp crest factor was undesireably increased because the rectified sine wave would not be smoothed out as much as with a larger capacitor.
To solve some of these problems, the inventor of the present invention developed the High Frequency, Electronic Fluorescent Ballast disclosed in co-pending application Ser. No. 077,760 now U.S. Pat. No. 4,870,327.
Briefly, the input power was converted to direct current in a power converter which provided direct current filament power directly to the lamp and low frequency alternating current to power the lamp via a commutator circuit. A control circuit controlled open circuit voltage and peak current to precise level by pulse width modulation of the power converter.
While the prior invention significantly advanced the state of the art, it did not address a major problem which was how to use the same ballast for different fluorescent lamps of varying lengths and wattages.
To provide a "universal" ballast, ay power converter must be capable of producing a wide range of output voltages at full output current. Preferably, the dynamic characteristics of the power converter should not change appreciably with changes of the output voltage over its entire range. In particular, there should be no shifts from discontinuous to continuous conduction in the power converter as the output voltage requirement drops with shorter lamps.
Further, it is desireable that there be some type of sequencing be provided to assure that full filament power is applied for a controlled period to heat the filaments before the high starting voltage is applied to start the lamp. This is required to prevent premature burn out of the filaments due to cold starts.
A universal ballast needs to have a provision for providing whatever minimum voltage is required to start any lamp regardless of length. Since gas discharge lamps have reverse resistance characteristics, over voltage conditions could lead to catastrophic failure of the lamps if the starting voltage continues to increase after the lamp starts. No such ballasts have heretofore been developed.
To assure maximum lamp life regardless of lamp length, the crest factor of lamp current (the ratio of peak current to RMS current) should be low, and preferably below 1.6.
In addition to a low crest factor, the power factor should be high, preferably above 0.85. However, even when the internal filter energy storage is kept as low as possible consistant with minimizing EMI, previous ballasts have either had low crest factor and low power factor or high crest factor and high power factor.
Still further, a universal ballast should have the peak current and average current track so that each provides the same percentage of their respective full output simultaneously, regardless of output voltage. This is to assure good power factor (good tracking of current and voltage) and crest factor simultaneously rather than the tradeoff which always occurred with the prior art as described above.
Finally, there should be a mechanism for reducing output power when different conditions require. This would include the installation of lamps which are outside the original design specifications of the ballast, ambient temperatures which exceed the proper operating conditions, or even automatic dimming when exterior light conditions indicate that full lighting is no longer required.